For the InstructorThese student materials complement the Renewable Energy and Environmental Sustainability Instructor Materials. If you would like your students to have access to the student materials, we suggest you either point them at the Student Version which omits the framing pages with information designed for faculty (and this box). Or you can download these pages in several formats that you can include in your course website or local Learning Managment System. Learn more about using, modifying, and sharing InTeGrate teaching materials.
Student Reading: Better Ways to Illuminate
Three common types of lamps used to illuminate are incandescent, fluorescent, and light-emitting diodes (LEDs). In this activity, students will compare the amount of heat and light generated by these lamps, assess their cost differences, and calculate the payback period.
Learning Goals: Students will be able to:
- Describe the similarities and differences among incandescent, fluorescent and LED lamps.
- Collect and compare data on the amount of light and heat generated by incandescent, fluorescent, and LED lamps.
- Determine the cost differences in operating incandescent, fluorescent, and LED lighting, and calculate their payback periods.
- Describe factors inhibiting the widespread adoption of compact fluorescent and LED lighting.
The incandescent lamp has been in use since the late 1800s, and although it is not a very efficient bulb, it is affordable and easy to produce. The light from an incandescent lamp is generated when a small filament of wire (made of tungsten) is heated to a high temperature (~ 3,000 to 5,500 ⁰F), begins to glow and thereby emits light. The filament of wire is contained in the bulb that is filled with inert gases argon and nitrogen. Since its invention, the incandescent bulb has undergone several changes that have made it more efficient. For example, the early filaments were made of carbon or other materials and did not last long before burning out. When this filament breaks, the bulb is blown, thus making a slight sound when you shake it. The Energy and Independence Act of 2007 required that incandescent lamps be more energy efficient so they have been replaced by incandescent halogen lamps. Halogen lamps are more efficient because they contain an inert gas (either Iodine or Bromine) which allows the tungsten filament to reach higher temperatures (i.e. produce more light). Also the gas assists in redepositing tungsten back onto the filament giving the lamp a longer life.
The typical tube shaped fluorescent lamp has been modified and now it can fit into small fixtures. These are called compact fluorescent lights and they produce light because of the coating of phosphor along the inside of the bulb. The familiar tube of a fluorescent lamp is filled with an inert gas like argon or neon plus a small amount of mercury. When an electrical current is applied, it excites the mercury electrons, which causes them to produce light. However, because much of the light produced is actually ultraviolet, it is not visible to the human eye. That is why the tube is coated with phosphor"it will emit visible light. Because CFLs contain mercury, they need to be handled properly if they break and disposed of properly; do not just throw them away in the garbage!
When you go shopping for these types of lights (e.g. LED) rather than thinking about the wattage start thinking about the amount of light (i.e., lumens) you need. For example, to replace a 60W lamp you would look for a lamp that emitted about 800 lumens. New lamps are sold with a label that will state the brightness (in lumens), estimated annual operating costs, the life of the bulb (an estimate), its energy use (in watts) and the light appearance. The light appearance ranges from warm (2700K) which provides a warm soft white light to cool (6500K) which provides a natural or daylight appearance.
Using the number of lumens and watts one can calculate the efficacy of a lamp by dividing the number of lumens by watts to get lumens per watt (LPW). Lamps that have have a higher efficacy provide more light and don't use as much energy as lamps with a lower efficacy therefore you will get more for you dollar using the higher efficacy bulb. For example, the efficacy of a incandescent lamp (see the lighting facts label) rated at 750 lumens and 43 W would be 17.4 LPW but for an LED lamp rated at 800 lumens and 9.W would be it would be 84.2 LPW.
A common way of measuring how cost-efficient it is to purchase an item or, in this case, an energy-efficient appliance, is to calculate the payback period. That is how long it would take you to recover the costs of that purchase (the shorter the payback period, the better the investment). It is the amount of time it takes for the cost savings of an energy-efficient appliance to cover its purchase, installation, and operating costs.
It equals the cost of the energy-efficient appliance minus the cost of the inefficient appliance divided by the annual savings. The annual savings is equal to the annual energy cost of operating the inefficient appliance minus the annual energy costs of the efficient appliance.
You are living in a home where the heat pump is broken and needs to be replaced. Because you keep track of your home's energy use, you know that the annual energy costs for the existing furnace is $1,800. Now you want to know if it would be a good idea to replace the existing heat pump with an energy-efficient model. You know that the annual energy cost for the new energy-efficient furnace is $1,500, and that you plan to be in your home at least another fifteen years. The cost of purchasing a new high efficiency heat pump is about $3,000. The cost of an inefficient heat pump is $1,000.
To calculate the payback, use the following calculations:
First Cost = Incremental cost of purchasing the new furnace
= Cost of high efficiency furnace - Cost of inefficient furnace
= $3,000 - $1,000
Annual Savings = Annual Energy Cost Existing Furnace - Annual Energy Cost of New Furnace
= $1,800 - $1,500
Payback Period = First Cost / Annual Savings
= $2,000 / $300
= 6.7 years
Collecting your thoughts: Systems thinking and reflection
You have just learned about three different approaches for artificial lighting. Take a few moments to consider how this all fits together. Think of each kind of lamp as a system. How are the sources of energy flowing into and out of each similar or different? Which system is the most efficient at converting electricity to light? Why? Traditional incandescent lamps are no longer sold in the United States so what are the pros and cons of choosing fluorescent versus LEDs as replacements?
The University of North Carolina is converting to LED lights, as described in this LEDsmagazine article: University of North Carolina plans transition to LED lighting. What do you think it would take to make the switch on your college campus?
Assessment: A comparison of incandescent, fluorescent, and light-emitting diode (LED) lamps.
Place each of the three types of lamps (incandescent, compact fluorescent, and LED) in into a test box. Plug the light into an outlet and record the light meter level and the temperature every ten minutes for forty minutes. Air out the box so it can cool to room temperature between bulb changes. You can use this handout to record results: Lab Sheet Handout (Microsoft Word 16kB Oct29 17)
1. Discuss the results from the above experiment. Which lamp do you think is the most efficient, and why? Rank them in terms of their ratio of light to temperature " how much more efficient is the best-performing bulb compared to the worst-performing bulb? Which bulb would be best to use in a house in which you were trying to reduce the cost of cooling, and why?
2. The cost of using any electrical appliance is its wattage multiplied by the price of electricity multiplied by the amount of time the appliance is used. Find out how much a it costs for a kilowatt hour in your area; calculate how much it costs to run the bulb for a year if it is on for five hours every day. Once you know how much it costs to operate the bulb for a year, determine the payback period for:
a) Replacing an incandescent bulb with an LED
b) Replacing a CFL with an LED
3. Based on the information you collected in questions one and two, why do you think LEDs and fluorescent bulbs are not more common in households?
References and Resources
- Electricity Work Power module
- Consumer Reports: How to read a lightbulb label, August 13, 2015
- EPA: Cleaning Up a Broken CFL
- EPA Energy Star: Learn about LED bulbs
- EPA Energy Star: Information on Compact Fluorescent Light Bulbs (CFLs) and Mercury
- Lux Magazine: Efficacy Explained, May 2012
- Sciencing: How to calculate efficacy, April 2017
- National Geographic: U.S. Phase-Out of Incandescent Light Bulbs Continues in 2014 with 40-, 60-Watt Varieties, December 2013
- US Department of Energy: Lighting Choices to Save You Money